A piezoelectric thin-film resonator, which is one of the acoustic wave devices, is used in, for example, a filter and a duplexer of wireless devices such as cellular phones. The piezoelectric thin-film resonator has a structure in which a lower electrode and an upper electrode face each other through a piezoelectric film.
Due to the rapid spread of wireless systems, many frequency bands have been used. Thus, there is a growing demand for the improvement in the steepness of the skirt characteristics of filters and duplexers. A measure for improving the steepness of the skirt characteristics is to increase the Q value of the piezoelectric thin-film resonator. A factor that degrades the Q value of the piezoelectric thin-film resonator is a leakage of acoustic wave energy from the resonance region to the outside. For the improvement of the Q value, there is a proposal to position a raised annulus on a surface of the lower electrode or the upper electrode (see Japanese Laid-Open Patent Application No. 2006-109472, for example).
However, the structure proposed in the above application has a difficulty in effective suppression of the acoustic wave energy leaked to the outside from the resonance region and an insufficiency in the improvement of the Q value.